1. Field of the Invention
This invention relates to a process for the aromatization of alkanes to aromatics, specifically use of a base-exchanged zeolite with germanium in the crystalline framework and with a noble metal deposited on the zeolite for the aromatization of alkanes having six to twelve carbon atoms per molecule to produce aromatics, such as benzene, ethyl benzene, toluene and xylene.
2. Description of the Prior Art
Zeolite is a crystalline hydrated aluminosilicate that may also contain other elements in the crystalline framework and/or deposited on its surface. The term “zeolite” includes not only aluminosilicates but substances in which the aluminum is replaced by other trivalent elements and substances in which silicon is replaced by other tetravalent elements. Generally, zeolites are structures of TO4 tetrahedra, which form a three dimensional network by sharing oxygen atoms where T represents tetravalent elements, such as silicon, and trivalent elements, such as aluminum.
A zeolite may be prepared by (a) preparing an aqueous mixture of silicon oxide, aluminum oxide and, optionally, oxides of other trivalent or tetravalent elements; and (b) maintaining said aqueous mixture under crystallization conditions until crystals of said zeolite form. The reaction mixture gel is heated and stirred to form zeolite crystals and then cooled. The zeolite crystals are separated from the gel and are washed, dried and calcined. Elements may be deposited on the zeolite by any means known in the art, for example, metals deposited by impregnation or ion exchange.
Aromatization of alkanes is a multi-step process of dehydrogenation of the alkane, cyclization of the dehydrogenated alkane and aromatization of the cyclized alkane. The catalyst for this process must be multi-functional to have an acceptable conversion and selectivity for the desired products. Zeolites are known catalysts for isomerization, toluene disproportionation, transalkylation, hydrogenation and alkane oligomerization and aromatization. Some zeolite catalysts, especially those containing a Group VIII deposited metal, are susceptible to sulfur poisoning.
U.S. Pat. No. 5,358,631 discloses a process for catalytic reforming or dehydrocyclization of hydrocarbons using a catalyst of a noble metal on an intermediate pore size crystalline silicate having a high silica to alumina ratio (greater than 200) and a relatively low alkali content (less than 6000 ppm). The patented catalyst has sulfur tolerance up to 2 ppm. Low acidity of the catalyst is attained not by using large amounts of alkali but by low aluminum content with low amounts of alkali and/or use of alkaline earth metal, such as magnesium, in the catalyst. Germanium is disclosed as a promoter metal in a conventional reforming catalyst but not in the patented low alkali catalyst.
U.S. Pat. No. 4,652,360 discloses a catalyst of zeolite, preferably ZSM-5 or ZSM-22, on which a Group VIII metal, such as platinum, has been deposited and which has been base-exchanged with Group IA metal cations, such as sodium hydroxide, potassium chloride or cesium hydroxide, to lower or essentially eliminate, the base exchangeable acidic content of the catalyst composition. One example illustrates n-hexane dehydrocyclization using Pt/ZSM-5 catalysts with and without Cs base-exchange treatment. There is no disclosure of germanium in the catalyst.
U.S. Pat. No. 7,153,801 discloses a method of making a catalyst of a large pore zeolite impregnated with platinum and at least one organic ammonium halide of the formula N(R)4X where X is a halide and R is a substituted or unsubstituted carbon, chain molecule having 1-20 carbon atoms. The ammonium halide may be an acid halide and an ammonium hydride of the formula N(R′)4OH where R′ is hydrogen or a substituted or unsubstituted carbon chain molecule having 12-20 carbon atoms. The catalyst is a bound potassium L-type zeolite (KL zeolite) used to dehydrocyclize aliphatic hydrocarbons (C6-C8 petroleum naphtha) to produce aromatic hydrocarbons (benzene, toluene and xylenes).
U.S. Pat. No. 4,867,864 discloses a dehydrogenation/dehydrocyclization process with a non-acidic catalyst of zeolite beta and a dehydrogenation/hydrogenation metal, such as platinum. C2-C5 paraffins are dehydrogenated and C6-C12 paraffins are dehydrocyclized. The acid content has been reduced by ion exchange of aluminum with Group IA and/or IIA cations, preferably cesium. Hydrogen must be added during dehydrocyclizaton.
Catalysts of platinum deposited on potassium L-zeolite which has been alkaline earth-exchanged (magnesium, calcium, strontium and barium) were disclosed for aromatization of paraffins, especially hexanes and heptanes, in Aromatization of Hydrocarbons over Platinum Alkaline Earth Zeolites, T. R. Hughes, W. C. Buss, P. W. Tamm and R. L. Jacobson, Proceedings of 7th International Zeolite Conference, Tokyo, p. 725-732 (1986). This catalyst is extremely sensitive to poisoning by sulfur.
The Aromax® Process selectively converts C6-C7 paraffins to high octane aromatics utilizing a platinum supported L-type zeolite catalyst of low acidity. A relatively high amount of hydrogen co-feed is required. The Pt/KL zeolite catalyst is sulfur-sensitive. The sulfur level in the feed must be controlled to low levels so that the catalyst is not deactivated. Octane Enhancement by Selective Reforming of Light Paraffins, P. W. Tamm, D. H. Mohr, and C. R. Wilson, Catalysis 1987, J. W. Ward (Editor), p. 335-353 (1988). Selective Catalytic Process for Conversion of Light Naphtha to Aromatics, D. V. Law, P. W. Tamm and C. M. Detz, Energy Progress, vol. 7, no. 4, p. 215-222 (December, 1987).
U.S. Pat. No. 4,517,306 discloses a catalyst for reforming paraffins containing at least 6 carbon atoms into corresponding aromatic hydrocarbons. The catalyst is a type L zeolite, an alkaline earth metal and a Group VIII metal which has been reduced with hydrogen. One essential element of the catalyst was the presence of the alkaline earth metal which must be barium, strontium or calcium, preferably barium since it lessens the acidity of the catalyst.
U.S. Pat. No. 4,104,320 discloses a method of dehydrocyclizing aliphatic hydrocarbons in the presence of hydrogen to form corresponding aromatic hydrocarbons with a catalyst of a type L zeolite which has at least 90% alkali metal (sodium, lithium, potassium, rubidium and cesium) exchangeable cations and contains a group VIII dehydrogenating metal and, optionally tin and/or germanium. No example containing germanium was prepared.
U.S. Pat. No. 4,417,083 discloses a process for production of aromatic hydrocarbons from petroleum fractions containing paraffins in the presence of hydrogen and a catalyst of noble metals and, optionally, sulfur deposited on a crystalline zeolitic aluminosilicate, such as zeolite L, having a pore size of larger than 6.5 Angstroms and substituted with more than 90% alkali metal cations, such as potassium. It is disclosed that the catalyst can contain iridium, tin or germanium in the range of 0-1.5% but no example of a catalyst containing germanium is disclosed.
U.S. Pat. No. 4,435,283 discloses a method of dehydrocyclizing alkanes, such as n-hexane, with a catalyst of a large-pore zeolite, such as type L zeolite, a Group VIII metal, such as platinum, and an alkaline earth metal (barium, strontium or calcium). Selectivity for n-hexane dehydrocyclization is greater than 60%. The feedstock is substantially free of sulfur and other known poisons for reforming catalysts.
U.S. Pat. No. 6,063,724 discloses a sulfur-tolerant Pt/KL-zeolite aromatization catalyst. The catalyst has a rare earth ion, such as lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium or lutetium, incorporated with the L-zeolite. Incorporation of the rare earth ion is by ion exchange, incipient wetness impregnation, chemical vapor deposition or other methods for dispersion of the ions and is after calcination of the L-zeolite.
Deactivation of the Pt/KL-zeolite catalyst in hexane aromatization appears to occur by agglomeration of the platinum and blockage of the zeolite channels. Sulfur accelerates platinum agglomeration and reduces the number of accessible catalytic sites but does not appear to modify activity and selectivity of the catalytic sites. Effect of Sulfur on the Performance and on the Particle Size and Location of Platinum in Pt/K Hexane Aromatization Catalysts, G. B. McVicker, J. L Kao, J. Ziemiak, W. E. Gates, J. L. Robbins, M. M. J. Treacy, S. N. Rice, T. H. Vanderspurt, V. R. Cross and A. K. Ghosh, Journal of Catalysis, vol. 139, p. 46-61 (1993).
It would be advantageous to have a sulfur-tolerant catalyst for aromatization of alkanes with high selectivity to aromatics, such as benzene.